Basic problems in collagen or bone physiology are being more clearly defined. The objective of these studies is to determine how the body handles non-mineralized and mineralized collagens at a physiological and biochemical level during various ages. The experimental design is to study the fate and replacement of collagen quantitatively in chronically labeled rats and dogs over long intervals of time after labeling and when the animals have reached a steady state, isotopically and biologically. The comparative turnover of pre-existing calcified and noncalcified collagen from endochondral (long) bones and membranous (calvaria) bones were evaluated for rats and dogs. Simultaneously, the increase in their collagen masses were measured with age. The kinetics of bone remodelling in vivo were studied in dogs and rats to quantify the differences between relative and absolute rates of bone resorption. A new method using H3-tetracycline was compared to Ca45 kinetics in whole bones of the prelabeled animals. The administration of large amounts of nonradioactive tetracycline to rats or dogs prelabeled with H3- tetracycline did not increase the loss of radioactivity from their bones nor decrease the rate of mineralization. Thus, the reuse of 3H- tetracycline did not appear to be a significant factor in the turnover of prelabeled bone.